题目
表： Users
sql
+----------------+---------+
| Column Name | Type |
+----------------+---------+
| user_id | int |
| join_date | date |
| favorite_brand | varchar |
+----------------+---------+
user_id 是此表主键（具有唯一值的列）。
表中描述了购物网站的用户信息，用户可以在此网站上进行商品买卖。表： Orders
sql
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| order_id | int |
| order_date | date |
| item_id | int |
| buyer_id | int |
| seller_id | int |
+---------------+---------+
order_id 是此表主键（具有唯一值的列）。
item_id 是 Items 表的外键（reference 列）。
（buyer_id，seller_id）是 User 表的外键。表：Items
sql
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| item_id | int |
| item_brand | varchar |
+---------------+---------+
item_id 是此表的主键（具有唯一值的列）。编写解决方案找出每个用户的注册日期和在 2019 年作为买家的订单总数。
以 任意顺序 返回结果表。
查询结果格式如下。
示例 1:
sql
输入：
Users 表:
+---------+------------+----------------+
| user_id | join_date | favorite_brand |
+---------+------------+----------------+
| 1 | 2018-01-01 | Lenovo |
| 2 | 2018-02-09 | Samsung |
| 3 | 2018-01-19 | LG |
| 4 | 2018-05-21 | HP |
+---------+------------+----------------+
Orders 表:
+----------+------------+---------+----------+-----------+
| order_id | order_date | item_id | buyer_id | seller_id |
+----------+------------+---------+----------+-----------+
| 1 | 2019-08-01 | 4 | 1 | 2 |
| 2 | 2018-08-02 | 2 | 1 | 3 |
| 3 | 2019-08-03 | 3 | 2 | 3 |
| 4 | 2018-08-04 | 1 | 4 | 2 |
| 5 | 2018-08-04 | 1 | 3 | 4 |
| 6 | 2019-08-05 | 2 | 2 | 4 |
+----------+------------+---------+----------+-----------+
Items 表:
+---------+------------+
| item_id | item_brand |
+---------+------------+
| 1 | Samsung |
| 2 | Lenovo |
| 3 | LG |
| 4 | HP |
+---------+------------+
输出：
+-----------+------------+----------------+
| buyer_id | join_date | orders_in_2019 |
+-----------+------------+----------------+
| 1 | 2018-01-01 | 1 |
| 2 | 2018-02-09 | 2 |
| 3 | 2018-01-19 | 0 |
| 4 | 2018-05-21 | 0 |
+-----------+------------+----------------+分析
只需要两张表，对用户id分组，过滤2019年订单，可以使用year('2019-05-02')或left('2019-05-02', 4)都可以获取2019，但是有两个用户没有订单，所以连表时缺少两条数据，解决方法如下：
外连接时要注意where和on的区别，on是在连接构造临时表时执行的，不管on中条件是否成立都会返回主表（也就是left join左边的表）的内容，where是在临时表形成后执行筛选作用的，不满足条件的整行都会被过滤掉。如果这里用的是 where year(order_date)='2019' 那么得到的结果将会把不满足条件的user_id为3，4的行给删掉。用on的话会保留user_id为3，4的行。
select user_id as buyer_id, join_date, count(order_id) as orders_in_2019
from Users as u left join Orders as o on u.user_id = o.buyer_id and year(order_date)='2019'
group by user_id
或者可以先用where对Orders表进行筛选
select user_id as buyer_id, join_date, count(order_id) as orders_in_2019
from Users as u left join (select * from Orders where year(order_date)='2019') as o on u.user_id = o.buyer_id
group by user_id
题解
sql
select u.user_id as buyer_id , u.join_date , count(o.order_id) as orders_in_2019 from users u left join orders o on u.user_id = o.buyer_id and year(o.order_date) = '2019' group by u.user_id
# year(o.order_date) = '2019' left(o.order_date, 4) = '2019'使用ifnull
sql
# 分步子查询，left join连接，使用ifnull列出不能匹配的数据
select Users.user_id as buyer_id, join_date, ifnull(UserBuy.cnt, 0) as orders_in_2019
from Users
left join (
# 找到2019年订单的id和数量
select buyer_id, count(order_id) cnt
from Orders
where order_date between '2019-01-01' and '2019-12-31'
group by buyer_id
) UserBuy
on Users.user_id = UserBuy.buyer_id